FIG. 1 shows a photograph of a typical gas turbine blade 10 in a perspective view. The turbine blade 10 comprises an airfoil 11, which extends in radial direction from a shaft and ends with a blade tip 14. Below the shaft 12 a foot 13 of the fir-tree type is provided for mounting the blade in a respective groove of the rotor. Part of the blade tip 14 is a crown (16 in FIG. 4), a circumferential rim. FIG. 2 is a side view of the blade 10. FIG. 3 is a top view. Further features of the blade tip 14 are tip cooling air holes 17 and so-called letterboxes 15, slots in the tip cover plate (25 in FIG. 4), which are closed by brazed closing plates (26 in FIG. 4).
The tip of a moving turbine blade is often prone to damages, which have to be repaired in order to restore the efficiency of the turbine. A special problem is connected with such a repair when the blade is of the single crystal type.
Document US 2011/052386 A discloses a method for depositing material on a turbine airfoil having a tip wall extending past a tip cap, wherein the tip wall includes a first alloy with a single crystal microstructure. The method includes: depositing a second alloy on at least a portion of the tip wall to form a repair structure, wherein a high temperature oxidation resistance of the second alloy is greater than a high temperature oxidation resistance of the first alloy, and wherein the repair structure has a crystallographic orientation that is substantially the same as a crystallographic orientation of the tip wall.
Document EP 1 674 658 A1 furnishes a damaged gas turbine blade which has previously been in service, and which is made of a base metal. Any damaged material is removed from the damaged blade tip. The damaged blade tip is weld repaired with a nickel-base superalloy that is more resistant to oxidation resistance than is the base metal in the operating environment of the tip-repaired gas turbine blade. The method does not include any step of coating a lateral surface of the repaired blade tip with a non-ceramic coating after the step of weld repairing.
Document US 2010/050435 A1 discloses a gas turbine blade, a gas turbine blade airfoil repair method, and an airfoil corresponding to the product of the method. The airfoil extends radially from a platform and includes a tip having a tip edge radially distant from the platform; a trailing edge; and a leading edge. The method includes forming a joining plane extending from the leading edge to the tip edge, wherein the joining plane forms at least one arc; forming a replacement tip fittable on the joining plane; and bonding the replacement tip to the airfoil.
In document EP 1 701 004 A1 a repaired gas turbine blade has a monocrystalline airfoil made of a first nickel-base superalloy, and a repair squealer tip welded to a tip of the airfoil. The repair squealer tip is made of a second nickel-base superalloy different from the first nickel-base superalloy and having less than about 0.15 weight percent total of carbon, boron, silicon, zirconium, and hafnium.
Document U.S. Pat. No. 6,332,272 B1 discloses a method for repairing a turbine blade wherein the tip of the blade is removed and a replacement cap is attached by welding. The cap may consist of a plate attached by welding and a squealer formed by depositing weld material. The plate and/or squealer may be formed from a material different from the material of the airfoil portion of the blade in order to optimize the performance of the blade.
The various known repairing methods for blade tips have different disadvantages.
For some methods there is not enough remaining wall thickness and attachment points for crown plate replacement, resulting in a high risk of failure (high temperature, high load on tip of the first blades). A full letterbox removal for crown plate replacement would cause either a dimensional change of the repaired blade or cause complicated repair effort to keep original dimensions.
A restoration of blade crown height by LMF (Laser Metal Forming) and subsequent crack brazing of letterboxes is not applicable in other cases for following reasons: The method works only for lightly damaged parts, combined welding and brazing would massively exceed any reconditioning cost restrictions.
An EDM removal of damaged tip letterbox and replacement with new brazed-in letterbox is not applicable in other cases for following reasons: A letterbox replacement has high risk of NCR and is an instable process, there is no solution for crown replacement, and the high costs exceed reconditioning cost restrictions for such a component.
The method for replacing blade tips of directionally solidified and single crystal turbine blades according to document U.S. Pat. No. 5,822,852 with a replacement of the squealer tip by brazing or welding is not applicable in other cases for following reasons: There is no solution for damaged ex-service letterbox repair, there is problematic handling of tip cooling air holes with squealer tip replacement, and damages below squealer tip level cannot be repaired.